Doc/whatsnew/3.3.rst

****************************
  What's New In Python 3.3
****************************

:Author: Raymond Hettinger
:Release: |release|
:Date: |today|

.. Rules for maintenance:

   * Anyone can add text to this document.  Do not spend very much time
   on the wording of your changes, because your text will probably
   get rewritten to some degree.

   * The maintainer will go through Misc/NEWS periodically and add
   changes; it's therefore more important to add your changes to
   Misc/NEWS than to this file.

   * This is not a complete list of every single change; completeness
   is the purpose of Misc/NEWS.  Some changes I consider too small
   or esoteric to include.  If such a change is added to the text,
   I'll just remove it.  (This is another reason you shouldn't spend
   too much time on writing your addition.)

   * If you want to draw your new text to the attention of the
   maintainer, add 'XXX' to the beginning of the paragraph or
   section.

   * It's OK to just add a fragmentary note about a change.  For
   example: "XXX Describe the transmogrify() function added to the
   socket module."  The maintainer will research the change and
   write the necessary text.

   * You can comment out your additions if you like, but it's not
   necessary (especially when a final release is some months away).

   * Credit the author of a patch or bugfix.   Just the name is
   sufficient; the e-mail address isn't necessary.

   * It's helpful to add the bug/patch number as a comment:

   XXX Describe the transmogrify() function added to the socket
   module.
   (Contributed by P.Y. Developer in :issue:`12345`.)

   This saves the maintainer the effort of going through the Mercurial log
   when researching a change.

This article explains the new features in Python 3.3, compared to 3.2.


Summary -- Release highlights
=============================

.. This section singles out the most important changes in Python 3.3.
   Brevity is key.

New syntax features:

* New ``yield from`` expression for :ref:`generator delegation <pep-380>`.
* The ``u'unicode'`` syntax is accepted again for :class:`str` objects.

New library modules:

* :mod:`faulthandler` (helps debugging low-level crashes)
* :mod:`ipaddress` (high-level objects representing IP addresses and masks)
* :mod:`lzma` (compress data using the XZ / LZMA algorithm)
* :mod:`unittest.mock` (replace parts of your system under test with mock objects)
* :mod:`venv` (Python :ref:`virtual environments <pep-405>`, as in the
  popular ``virtualenv`` package)

New built-in features:

* Reworked :ref:`I/O exception hierarchy <pep-3151>`.

Implementation improvements:

* Rewritten :ref:`import machinery <importlib>` based on :mod:`importlib`.
* More compact :ref:`unicode strings <pep-393>`.
* More compact :ref:`attribute dictionaries <pep-412>`.

Security improvements:

* Hash randomization is switched on by default.

Please read on for a comprehensive list of user-facing changes.


.. _pep-405:

PEP 405: Virtual Environments
=============================

:pep:`405` - Python Virtual Environments
 PEP written by Carl Meyer, implemented by Carl Meyer and Vinay Sajip.

Virtual environments help create separate Python setups while sharing a
system-wide base install, for ease of maintenance.  Virtual environments
have their own set of private site packages (i.e. locally-installed
libraries), and are optionally segregated from the system-wide site
packages.  Their concept and implementation are inspired by the popular
``virtualenv`` third-party package, but benefit from tighter integration
with the interpreter core.

This PEP adds the :mod:`venv` module for programmatic access, and the
:ref:`pyvenv <scripts-pyvenv>` script for command-line access and
administration.  The Python interpreter becomes aware of a ``pvenv.cfg``
file whose existence signals the base of a virtual environment's directory
tree.


PEP 420: Namespace Packages
===========================

Native support for package directories that don't require ``__init__.py``
marker files and can automatically span multiple path segments (inspired by
various third party approaches to namespace packages, as described in
:pep:`420`)


.. _pep-3118-update:

PEP 3118: New memoryview implementation and buffer protocol documentation
=========================================================================

:issue:`10181` - memoryview bug fixes and features.
  Written by Stefan Krah.

The new memoryview implementation comprehensively fixes all ownership and
lifetime issues of dynamically allocated fields in the Py_buffer struct
that led to multiple crash reports. Additionally, several functions that
crashed or returned incorrect results for non-contiguous or multi-dimensional
input have been fixed.

The memoryview object now has a PEP-3118 compliant getbufferproc()
that checks the consumer's request type. Many new features have been
added, most of them work in full generality for non-contiguous arrays
and arrays with suboffsets.

The documentation has been updated, clearly spelling out responsibilities
for both exporters and consumers. Buffer request flags are grouped into
basic and compound flags. The memory layout of non-contiguous and
multi-dimensional NumPy-style arrays is explained.

Features
--------

* All native single character format specifiers in struct module syntax
  (optionally prefixed with '@') are now supported.

* With some restrictions, the cast() method allows changing of format and
  shape of C-contiguous arrays.

* Multi-dimensional list representations are supported for any array type.

* Multi-dimensional comparisons are supported for any array type.

* One-dimensional memoryviews of hashable (read-only) types with formats B,
  b or c are now hashable. (Contributed by Antoine Pitrou in :issue:`13411`)

* Arbitrary slicing of any 1-D arrays type is supported. For example, it
  is now possible to reverse a memoryview in O(1) by using a negative step.

API changes
-----------

* The maximum number of dimensions is officially limited to 64.

* The representation of empty shape, strides and suboffsets is now
  an empty tuple instead of None.

* Accessing a memoryview element with format 'B' (unsigned bytes)
  now returns an integer (in accordance with the struct module syntax).
  For returning a bytes object the view must be cast to 'c' first.

* memoryview comparisons now use the logical structure of the operands
  and compare all array elements by value. All format strings in struct
  module syntax are supported. Views with unrecognised format strings
  are still permitted, but will always compare as unequal, regardless
  of view contents.

* For further changes see `Build and C API Changes`_ and `Porting C code`_ .

.. _pep-393:

PEP 393: Flexible String Representation
=======================================

The Unicode string type is changed to support multiple internal
representations, depending on the character with the largest Unicode ordinal
(1, 2, or 4 bytes) in the represented string.  This allows a space-efficient
representation in common cases, but gives access to full UCS-4 on all
systems.  For compatibility with existing APIs, several representations may
exist in parallel; over time, this compatibility should be phased out.

On the Python side, there should be no downside to this change.

On the C API side, PEP 393 is fully backward compatible.  The legacy API
should remain available at least five years.  Applications using the legacy
API will not fully benefit of the memory reduction, or - worse - may use
a bit more memory, because Python may have to maintain two versions of each
string (in the legacy format and in the new efficient storage).

Functionality
-------------

Changes introduced by :pep:`393` are the following:

* Python now always supports the full range of Unicode codepoints, including
  non-BMP ones (i.e. from ``U+0000`` to ``U+10FFFF``).  The distinction between
  narrow and wide builds no longer exists and Python now behaves like a wide
  build, even under Windows.

* With the death of narrow builds, the problems specific to narrow builds have
  also been fixed, for example:

  * :func:`len` now always returns 1 for non-BMP characters,
    so ``len('\U0010FFFF') == 1``;

  * surrogate pairs are not recombined in string literals,
    so ``'\uDBFF\uDFFF' != '\U0010FFFF'``;

  * indexing or slicing non-BMP characters returns the expected value,
    so ``'\U0010FFFF'[0]`` now returns ``'\U0010FFFF'`` and not ``'\uDBFF'``;

  * all other functions in the standard library now correctly handle
    non-BMP codepoints.

* The value of :data:`sys.maxunicode` is now always ``1114111`` (``0x10FFFF``
  in hexadecimal).  The :c:func:`PyUnicode_GetMax` function still returns
  either ``0xFFFF`` or ``0x10FFFF`` for backward compatibility, and it should
  not be used with the new Unicode API (see :issue:`13054`).

* The :file:`./configure` flag ``--with-wide-unicode`` has been removed.

Performance and resource usage
------------------------------

The storage of Unicode strings now depends on the highest codepoint in the string:

* pure ASCII and Latin1 strings (``U+0000-U+00FF``) use 1 byte per codepoint;

* BMP strings (``U+0000-U+FFFF``) use 2 bytes per codepoint;

* non-BMP strings (``U+10000-U+10FFFF``) use 4 bytes per codepoint.

The net effect is that for most applications, memory usage of string
storage should decrease significantly - especially compared to former
wide unicode builds - as, in many cases, strings will be pure ASCII
even in international contexts (because many strings store non-human
language data, such as XML fragments, HTTP headers, JSON-encoded data,
etc.).  We also hope that it will, for the same reasons, increase CPU
cache efficiency on non-trivial applications. The memory usage of
Python 3.3 is two to three times smaller than Python 3.2, and a little
bit better than Python 2.7, on a Django benchmark (see the PEP for
details).


.. _pep-3151:

PEP 3151: Reworking the OS and IO exception hierarchy
=====================================================

:pep:`3151` - Reworking the OS and IO exception hierarchy
 PEP written and implemented by Antoine Pitrou.

The hierarchy of exceptions raised by operating system errors is now both
simplified and finer-grained.

You don't have to worry anymore about choosing the appropriate exception
type between :exc:`OSError`, :exc:`IOError`, :exc:`EnvironmentError`,
:exc:`WindowsError`, :exc:`mmap.error`, :exc:`socket.error` or
:exc:`select.error`.  All these exception types are now only one:
:exc:`OSError`.  The other names are kept as aliases for compatibility
reasons.

Also, it is now easier to catch a specific error condition.  Instead of
inspecting the ``errno`` attribute (or ``args[0]``) for a particular
constant from the :mod:`errno` module, you can catch the adequate
:exc:`OSError` subclass.  The available subclasses are the following:

* :exc:`BlockingIOError`
* :exc:`ChildProcessError`
* :exc:`ConnectionError`
* :exc:`FileExistsError`
* :exc:`FileNotFoundError`
* :exc:`InterruptedError`
* :exc:`IsADirectoryError`
* :exc:`NotADirectoryError`
* :exc:`PermissionError`
* :exc:`ProcessLookupError`
* :exc:`TimeoutError`

And the :exc:`ConnectionError` itself has finer-grained subclasses:

* :exc:`BrokenPipeError`
* :exc:`ConnectionAbortedError`
* :exc:`ConnectionRefusedError`
* :exc:`ConnectionResetError`

Thanks to the new exceptions, common usages of the :mod:`errno` can now be
avoided.  For example, the following code written for Python 3.2::

    from errno import ENOENT, EACCES, EPERM

    try:
        with open("document.txt") as f:
            content = f.read()
    except IOError as err:
        if err.errno == ENOENT:
            print("document.txt file is missing")
        elif err.errno in (EACCES, EPERM):
            print("You are not allowed to read document.txt")
        else:
            raise

can now be written without the :mod:`errno` import and without manual
inspection of exception attributes::

    try:
        with open("document.txt") as f:
            content = f.read()
    except FileNotFoundError:
        print("document.txt file is missing")
    except PermissionError:
        print("You are not allowed to read document.txt")


.. _pep-380:

PEP 380: Syntax for Delegating to a Subgenerator
================================================

:pep:`380` - Syntax for Delegating to a Subgenerator
 PEP written by Greg Ewing.

PEP 380 adds the ``yield from`` expression, allowing a generator to delegate
part of its operations to another generator. This allows a section of code
containing 'yield' to be factored out and placed in another generator.
Additionally, the subgenerator is allowed to return with a value, and the
value is made available to the delegating generator.

While designed primarily for use in delegating to a subgenerator, the ``yield
from`` expression actually allows delegation to arbitrary subiterators.

For simple iterators, ``yield from iterable`` is essentially just a shortened
form of ``for item in iterable: yield item``::

    >>> def g(x):
    ...     yield from range(x, 0, -1)
    ...     yield from range(x)
    ...
    >>> list(g(5))
    [5, 4, 3, 2, 1, 0, 1, 2, 3, 4]

However, unlike an ordinary loop, ``yield from`` allows subgenerators to
receive sent and thrown values directly from the calling scope, and
return a final value to the outer generator::

    >>> def accumulate(start=0):
    ...     tally = start
    ...     while 1:
    ...         next = yield
    ...         if next is None:
    ...             return tally
    ...         tally += next
    ...
    >>> def gather_tallies(tallies, start=0):
    ...     while 1:
    ...         tally = yield from accumulate()
    ...         tallies.append(tally)
    ...
    >>> tallies = []
    >>> acc = gather_tallies(tallies)
    >>> next(acc) # Ensure the accumulator is ready to accept values
    >>> for i in range(10):
    ...     acc.send(i)
    ...
    >>> acc.send(None) # Finish the first tally
    >>> for i in range(5):
    ...     acc.send(i)
    ...
    >>> acc.send(None) # Finish the second tally
    >>> tallies
    [45, 10]

The main principle driving this change is to allow even generators that are
designed to be used with the ``send`` and ``throw`` methods to be split into
multiple subgenerators as easily as a single large function can be split into
multiple subfunctions.

(Implementation by Greg Ewing, integrated into 3.3 by Renaud Blanch, Ryan
Kelly and Nick Coghlan, documentation by Zbigniew Jędrzejewski-Szmek and
Nick Coghlan)


PEP 409: Suppressing exception context
======================================

:pep:`409` - Suppressing exception context
 PEP written by Ethan Furman, implemented by Ethan Furman and Nick Coghlan.

PEP 409 introduces new syntax that allows the display of the chained
exception context to be disabled. This allows cleaner error messages in
applications that convert between exception types::

    >>> class D:
    ...     def __init__(self, extra):
    ...         self._extra_attributes = extra
    ...     def __getattr__(self, attr):
    ...         try:
    ...             return self._extra_attributes[attr]
    ...         except KeyError:
    ...             raise AttributeError(attr) from None
    ...
    >>> D({}).x
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<stdin>", line 8, in __getattr__
    AttributeError: x

Without the ``from None`` suffix to suppress the cause, the original
exception would be displayed by default::

    >>> class C:
    ...     def __init__(self, extra):
    ...         self._extra_attributes = extra
    ...     def __getattr__(self, attr):
    ...         try:
    ...             return self._extra_attributes[attr]
    ...         except KeyError:
    ...             raise AttributeError(attr)
    ...
    >>> C({}).x
    Traceback (most recent call last):
      File "<stdin>", line 6, in __getattr__
    KeyError: 'x'

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<stdin>", line 8, in __getattr__
    AttributeError: x

No debugging capability is lost, as the original exception context remains
available if needed (for example, if an intervening library has incorrectly
suppressed valuable underlying details)::

    >>> try:
    ...     D({}).x
    ... except AttributeError as exc:
    ...     print(repr(exc.__context__))
    ...
    KeyError('x',)


PEP 414: Explicit Unicode literals
======================================

:pep:`414` - Explicit Unicode literals
 PEP written by Armin Ronacher.

To ease the transition from Python 2 for Unicode aware Python applications
that make heavy use of Unicode literals, Python 3.3 once again supports the
"``u``" prefix for string literals. This prefix has no semantic significance
in Python 3, it is provided solely to reduce the number of purely mechanical
changes in migrating to Python 3, making it easier for developers to focus on
the more significant semantic changes (such as the stricter default
separation of binary and text data).


PEP 3155: Qualified name for classes and functions
==================================================

:pep:`3155` - Qualified name for classes and functions
 PEP written and implemented by Antoine Pitrou.

Functions and class objects have a new ``__qualname__`` attribute representing
the "path" from the module top-level to their definition.  For global functions
and classes, this is the same as ``__name__``.  For other functions and classes,
it provides better information about where they were actually defined, and
how they might be accessible from the global scope.

Example with (non-bound) methods::

   >>> class C:
   ...     def meth(self):
   ...         pass
   >>> C.meth.__name__
   'meth'
   >>> C.meth.__qualname__
   'C.meth'

Example with nested classes::

   >>> class C:
   ...     class D:
   ...         def meth(self):
   ...             pass
   ...
   >>> C.D.__name__
   'D'
   >>> C.D.__qualname__
   'C.D'
   >>> C.D.meth.__name__
   'meth'
   >>> C.D.meth.__qualname__
   'C.D.meth'

Example with nested functions::

   >>> def outer():
   ...     def inner():
   ...         pass
   ...     return inner
   ...
   >>> outer().__name__
   'inner'
   >>> outer().__qualname__
   'outer.<locals>.inner'

The string representation of those objects is also changed to include the
new, more precise information::

   >>> str(C.D)
   "<class '__main__.C.D'>"
   >>> str(C.D.meth)
   '<function C.D.meth at 0x7f46b9fe31e0>'


.. _pep-412:

PEP 412: Key-Sharing Dictionary
===============================

:pep:`412` - Key-Sharing Dictionary
 PEP written and implemented by Mark Shannon.

Dictionaries used for the storage of objects' attributes are now able to
share part of their internal storage between each other (namely, the part
which stores the keys and their respective hashes).  This reduces the memory
consumption of programs creating many instances of non-builtin types.


PEP 362: Function Signature Object
==================================

:pep:`362`: -  Function Signature Object
 PEP written by Brett Cannon, Yury Selivanov, Larry Hastings, Jiwon Seo.
 Implemented by Yury Selivanov.

A new function :func:`inspect.signature` makes introspection of python
callables easy and straightforward.  A broad range of callables is supported:
python functions, decorated or not, classes, and :func:`functools.partial`
objects.  New classes :class:`inspect.Signature`, :class:`inspect.Parameter`
and :class:`inspect.BoundArguments` hold information about the call signatures,
such as, annotations, default values, parameters kinds, and bound arguments,
which considerably simplifies writing decorators and any code that validates
or amends calling signatures or arguments.


PEP 421: Adding sys.implementation
==================================

:pep:`421` - Adding sys.implementation
 PEP written and implemented by Eric Snow.

A new attribute on the :mod:`sys` module exposes details specific to the
implementation of the currently running interpreter.  The initial set of
attributes on :attr:`sys.implementation` are ``name``, ``version``,
``hexversion``, and ``cache_tag``.

The intention of ``sys.implementation`` is to consolidate into one namespace
the implementation-specific data used by the standard library.  This allows
different Python implementations to share a single standard library code base
much more easily.  In its initial state, ``sys.implementation`` holds only a
small portion of the implementation-specific data.  Over time that ratio will
shift in order to make the standard library more portable.

One example of improved standard library portability is ``cache_tag``.  As of
Python 3.3, ``sys.implementation.cache_tag`` is used by :mod:`importlib` to
support :pep:`3147` compliance.  Any Python implementation that uses
``importlib`` for its built-in import system may use ``cache_tag`` to control
the caching behavior for modules.

SimpleNamespace
---------------

The implementation of ``sys.implementation`` also introduces a new type to
Python: :class:`types.SimpleNamespace`.  In contrast to a mapping-based
namespace, like :class:`dict`, ``SimpleNamespace`` is attribute-based, like
:class:`object`.  However, unlike ``object``, ``SimpleNamespace`` instances
are writable.  This means that you can add, remove, and modify the namespace
through normal attribute access.


.. _importlib:

Using importlib as the Implementation of Import
===============================================
:issue:`2377` - Replace __import__ w/ importlib.__import__
:issue:`13959` - Re-implement parts of :mod:`imp` in pure Python
:issue:`14605` - Make import machinery explicit
:issue:`14646` - Require loaders set __loader__ and __package__

(Written by Brett Cannon)

The :func:`__import__` function is now powered by :func:`importlib.__import__`.
This work leads to the completion of "phase 2" of :pep:`302`. There are
multiple benefits to this change. First, it has allowed for more of the
machinery powering import to be exposed instead of being implicit and hidden
within the C code. It also provides a single implementation for all Python VMs
supporting Python 3.3 to use, helping to end any VM-specific deviations in
import semantics. And finally it eases the maintenance of import, allowing for
future growth to occur.

For the common user, this change should result in no visible change in
semantics. Any possible changes required in one's code to handle this change
should read the `Porting Python code`_ section of this document to see what
needs to be changed, but it will only affect those that currently manipulate
import or try calling it programmatically.

New APIs
--------
One of the large benefits of this work is the exposure of what goes into
making the import statement work. That means the various importers that were
once implicit are now fully exposed as part of the :mod:`importlib` package.

The abstract base classes defined in :mod:`importlib.abc` have been expanded
to properly delineate between :term:`meta path finders <meta path finder>`
and :term:`path entry finders <path entry finder>` by introducing
:class:`importlib.abc.MetaPathFinder` and
:class:`importlib.abc.PathEntryFinder`, respectively. The old ABC of
:class:`importlib.abc.Finder` is now only provided for backwards-compatibility
and does not enforce any method requirements.

In terms of finders, :class:`importlib.machinery.FileFinder` exposes the
mechanism used to search for source and bytecode files of a module. Previously
this class was an implicit member of :attr:`sys.path_hooks`.

For loaders, the new abstract base class :class:`importlib.abc.FileLoader` helps
write a loader that uses the file system as the storage mechanism for a module's
code. The loader for source files
(:class:`importlib.machinery.SourceFileLoader`), sourceless bytecode files
(:class:`importlib.machinery.SourcelessFileLoader`), and extension modules
(:class:`importlib.machinery.ExtensionFileLoader`) are now available for
direct use.

:exc:`ImportError` now has ``name`` and ``path`` attributes which are set when
there is relevant data to provide. The message for failed imports will also
provide the full name of the module now instead of just the tail end of the
module's name.

The :func:`importlib.invalidate_caches` function will now call the method with
the same name on all finders cached in :attr:`sys.path_importer_cache` to help
clean up any stored state as necessary.

Visible Changes
---------------
[For potential required changes to code, see the `Porting Python code`_
section]

Beyond the expanse of what :mod:`importlib` now exposes, there are other
visible changes to import. The biggest is that :attr:`sys.meta_path` and
:attr:`sys.path_hooks` now store all of the meta path finders and path entry
hooks used by import.  Previously the finders were implicit and hidden within
the C code of import instead of being directly exposed. This means that one can
now easily remove or change the order of the various finders to fit one's needs.

Another change is that all modules have a ``__loader__`` attribute, storing the
loader used to create the module. :pep:`302` has been updated to make this
attribute mandatory for loaders to implement, so in the future once 3rd-party
loaders have been updated people will be able to rely on the existence of the
attribute. Until such time, though, import is setting the module post-load.

Loaders are also now expected to set the ``__package__`` attribute from
:pep:`366`. Once again, import itself is already setting this on all loaders
from :mod:`importlib` and import itself is setting the attribute post-load.

``None`` is now inserted into :attr:`sys.path_importer_cache` when no finder
can be found on :attr:`sys.path_hooks`. Since :class:`imp.NullImporter` is not
directly exposed on :attr:`sys.path_hooks` it could no longer be relied upon to
always be available to use as a value representing no finder found.

All other changes relate to semantic changes which should be taken into
consideration when updating code for Python 3.3, and thus should be read about
in the `Porting Python code`_ section of this document.


New Email Package Features
==========================

Policy Framework
----------------

The email package now has a :mod:`~email.policy` framework.  A
:class:`~email.policy.Policy` is an object with several methods and properties
that control how the email package behaves.  The primary policy for Python 3.3
is the :class:`~email.policy.Compat32` policy, which provides backward
compatibility with the email package in Python 3.2.  A ``policy`` can be
specified when an email message is parsed by a :mod:`~email.parser`, or when a
:class:`~email.message.Message` object is created, or when an email is
serialized using a :mod:`~email.generator`.  Unless overridden, a policy passed
to a ``parser`` is inherited by all the ``Message`` object and sub-objects
created by the ``parser``.  By default a ``generator`` will use the policy of
the ``Message`` object it is serializing.  The default policy is
:data:`~email.policy.compat32`.

The minimum set of controls implemented by all ``policy`` objects are:

    ===============     =======================================================
    max_line_length     The maximum length, excluding the linesep character(s),
                        individual lines may have when a ``Message`` is
                        serialized.  Defaults to 78.

    linesep             The character used to separate individual lines when a
                        ``Message`` is serialized.  Defaults to ``\n``.

    cte_type            ``7bit`` or ``8bit``.  ``8bit`` applies only to a
                        ``Bytes`` ``generator``, and means that non-ASCII may
                        be used where allowed by the protocol (or where it
                        exists in the original input).

    raise_on_defect     Causes a ``parser`` to raise error when defects are
                        encountered instead of adding them to the ``Message``
                        object's ``defects`` list.
    ===============     =======================================================

A new policy instance, with new settings, is created using the
:meth:`~email.policy.Policy.clone` method of policy objects.  ``clone`` takes
any of the above controls as keyword arguments.  Any control not specified in
the call retains its default value.  Thus you can create a policy that uses
``\r\n`` linesep characters like this::

    mypolicy = compat32.clone(linesep='\r\n')

Policies can be used to make the generation of messages in the format needed by
your application simpler.  Instead of having to remember to specify
``linesep='\r\n'`` in all the places you call a ``generator``, you can specify
it once, when you set the policy used by the ``parser`` or the ``Message``,
whichever your program uses to create ``Message`` objects.  On the other hand,
if you need to generate messages in multiple forms, you can still specify the
parameters in the appropriate ``generator`` call.  Or you can have custom
policy instances for your different cases, and pass those in when you create
the ``generator``.


Provisional Policy with New Header API
--------------------------------------

While the policy framework is worthwhile all by itself, the main motivation for
introducing it is to allow the creation of new policies that implement new
features for the email package in a way that maintains backward compatibility
for those who do not use the new policies.  Because the new policies introduce a
new API, we are releasing them in Python 3.3 as a :term:`provisional policy
<provisional package>`.  Backwards incompatible changes (up to and including
removal of the code) may occur if deemed necessary by the core developers.

The new policies are instances of :class:`~email.policy.EmailPolicy`,
and add the following additional controls:

    ===============     =======================================================
    refold_source       Controls whether or not headers parsed by a
                        :mod:`~email.parser` are refolded by the
                        :mod:`~email.generator`.  It can be ``none``, ``long``,
                        or ``all``.  The default is ``long``, which means that
                        source headers with a line longer than
                        ``max_line_length`` get refolded.  ``none`` means no
                        line get refolded, and ``all`` means that all lines
                        get refolded.

    header_factory      A callable that take a ``name`` and ``value`` and
                        produces a custom header object.
    ===============     =======================================================

The ``header_factory`` is the key to the new features provided by the new
policies.  When one of the new policies is used, any header retrieved from
a ``Message`` object is an object produced by the ``header_factory``, and any
time you set a header on a ``Message`` it becomes an object produced by
``header_factory``.  All such header objects have a ``name`` attribute equal
to the header name.  Address and Date headers have additional attributes
that give you access to the parsed data of the header.  This means you can now
do things like this::

    >>> m = Message(policy=SMTP)
    >>> m['To'] = 'Éric <foo@example.com>'
    >>> m['to']
    'Éric <foo@example.com>'
    >>> m['to'].addresses
    (Address(display_name='Éric', username='foo', domain='example.com'),)
    >>> m['to'].addresses[0].username
    'foo'
    >>> m['to'].addresses[0].display_name
    'Éric'
    >>> m['Date'] = email.utils.localtime()
    >>> m['Date'].datetime
    datetime.datetime(2012, 5, 25, 21, 39, 24, 465484, tzinfo=datetime.timezone(datetime.timedelta(-1, 72000), 'EDT'))
    >>> m['Date']
    'Fri, 25 May 2012 21:44:27 -0400'
    >>> print(m)
    To: =?utf-8?q?=C3=89ric?= <foo@example.com>
    Date: Fri, 25 May 2012 21:44:27 -0400

You will note that the unicode display name is automatically encoded as
``utf-8`` when the message is serialized, but that when the header is accessed
directly, you get the unicode version.  This eliminates any need to deal with
the :mod:`email.header` :meth:`~email.header.decode_header` or
:meth:`~email.header.make_header` functions.

You can also create addresses from parts::

    >>> m['cc'] = [Group('pals', [Address('Bob', 'bob', 'example.com'),
    ...                           Address('Sally', 'sally', 'example.com')]),
    ...            Address('Bonzo', addr_spec='bonz@laugh.com')]
    >>> print(m)
    To: =?utf-8?q?=C3=89ric?= <foo@example.com>
    Date: Fri, 25 May 2012 21:44:27 -0400
    cc: pals: Bob <bob@example.com>, Sally <sally@example.com>;, Bonzo <bonz@laugh.com>

Decoding to unicode is done automatically::

    >>> m2 = message_from_string(str(m))
    >>> m2['to']
    'Éric <foo@example.com>'

When you parse a message, you can use the ``addresses`` and ``groups``
attributes of the header objects to access the groups and individual
addresses::

    >>> m2['cc'].addresses
    (Address(display_name='Bob', username='bob', domain='example.com'), Address(display_name='Sally', username='sally', domain='example.com'), Address(display_name='Bonzo', username='bonz', domain='laugh.com'))
    >>> m2['cc'].groups
    (Group(display_name='pals', addresses=(Address(display_name='Bob', username='bob', domain='example.com'), Address(display_name='Sally', username='sally', domain='example.com')), Group(display_name=None, addresses=(Address(display_name='Bonzo', username='bonz', domain='laugh.com'),))

In summary, if you use one of the new policies, header manipulation works the
way it ought to:  your application works with unicode strings, and the email
package transparently encodes and decodes the unicode to and from the RFC
standard Content Transfer Encodings.


Other Language Changes
======================

Some smaller changes made to the core Python language are:

* Added support for Unicode name aliases and named sequences.
  Both :func:`unicodedata.lookup()` and ``'\N{...}'`` now resolve name aliases,
  and :func:`unicodedata.lookup()` resolves named sequences too.

  (Contributed by Ezio Melotti in :issue:`12753`)

* Equality comparisons on :func:`range` objects now return a result reflecting
  the equality of the underlying sequences generated by those range objects.

  (:issue:`13201`)

* The ``count()``, ``find()``, ``rfind()``, ``index()`` and ``rindex()``
  methods of :class:`bytes` and :class:`bytearray` objects now accept an
  integer between 0 and 255 as their first argument.

  (Contributed by Petri Lehtinen in :issue:`12170`)

* New methods have been added to :class:`list` and :class:`bytearray`:
  ``copy()`` and ``clear()``.

  (:issue:`10516`)

* Raw bytes literals can now be written ``rb"..."`` as well as ``br"..."``.
  (Contributed by Antoine Pitrou in :issue:`13748`.)

* :meth:`dict.setdefault` now does only one lookup for the given key, making
  it atomic when used with built-in types.
  (Contributed by Filip Gruszczyński in :issue:`13521`.)


.. XXX mention new error messages for passing wrong number of arguments to functions


A Finer-Grained Import Lock
===========================

Previous versions of CPython have always relied on a global import lock.
This led to unexpected annoyances, such as deadlocks when importing a module
would trigger code execution in a different thread as a side-effect.
Clumsy workarounds were sometimes employed, such as the
:c:func:`PyImport_ImportModuleNoBlock` C API function.

In Python 3.3, importing a module takes a per-module lock.  This correctly
serializes importation of a given module from multiple threads (preventing
the exposure of incompletely initialized modules), while eliminating the
aforementioned annoyances.

(contributed by Antoine Pitrou in :issue:`9260`.)


Builtin functions and types
===========================

* :func:`open` gets a new *opener* parameter: the underlying file descriptor
  for the file object is then obtained by calling *opener* with (*file*,
  *flags*). It can be used to use custom flags like :data:`os.O_CLOEXEC` for
  example. The ``'x'`` mode was added: open for exclusive creation, failing if
  the file already exists.
* :func:`print`: added the *flush* keyword argument. If the *flush* keyword
  argument is true, the stream is forcibly flushed.
* :func:`hash`: hash randomization is enabled by default, see
  :meth:`object.__hash__` and :envvar:`PYTHONHASHSEED`.
* The :class:`str` type gets a new :meth:`~str.casefold` method: return a
  casefolded copy of the string, casefolded strings may be used for caseless
  matching. For example, ``'ß'.casefold()`` returns ``'ss'``.
* The sequence documentation has been substantially rewritten to better
  explain the binary/text sequence distinction and to provide specific
  documentation sections for the individual builtin sequence types
  (:issue:`4966`)

New Modules
===========

faulthandler
------------

This new debug module :mod:`faulthandler` contains functions to dump Python tracebacks explicitly,
on a fault (a crash like a segmentation fault), after a timeout, or on a user
signal. Call :func:`faulthandler.enable` to install fault handlers for the
:const:`SIGSEGV`, :const:`SIGFPE`, :const:`SIGABRT`, :const:`SIGBUS`, and
:const:`SIGILL` signals. You can also enable them at startup by setting the
:envvar:`PYTHONFAULTHANDLER` environment variable or by using :option:`-X`
``faulthandler`` command line option.

Example of a segmentation fault on Linux: ::

    $ python -q -X faulthandler
    >>> import ctypes
    >>> ctypes.string_at(0)
    Fatal Python error: Segmentation fault

    Current thread 0x00007fb899f39700:
      File "/home/python/cpython/Lib/ctypes/__init__.py", line 486 in string_at
      File "<stdin>", line 1 in <module>
    Segmentation fault


ipaddress
---------

The new :mod:`ipaddress` module provides tools for creating and manipulating
objects representing IPv4 and IPv6 addresses, networks and interfaces (i.e.
an IP address associated with a specific IP subnet).

(Contributed by Google and Peter Moody in :pep:`3144`)

lzma
----

The newly-added :mod:`lzma` module provides data compression and decompression
using the LZMA algorithm, including support for the ``.xz`` and ``.lzma``
file formats.

(Contributed by Nadeem Vawda and Per Øyvind Karlsen in :issue:`6715`)


Improved Modules
================

abc
---

Improved support for abstract base classes containing descriptors composed with
abstract methods. The recommended approach to declaring abstract descriptors is
now to provide :attr:`__isabstractmethod__` as a dynamically updated
property. The built-in descriptors have been updated accordingly.

  * :class:`abc.abstractproperty` has been deprecated, use :class:`property`
    with :func:`abc.abstractmethod` instead.
  * :class:`abc.abstractclassmethod` has been deprecated, use
    :class:`classmethod` with :func:`abc.abstractmethod` instead.
  * :class:`abc.abstractstaticmethod` has been deprecated, use
    :class:`staticmethod` with :func:`abc.abstractmethod` instead.

(Contributed by Darren Dale in :issue:`11610`)

array
-----

The :mod:`array` module supports the :c:type:`long long` type using ``q`` and
``Q`` type codes.

(Contributed by Oren Tirosh and Hirokazu Yamamoto in :issue:`1172711`)


base64, binascii
----------------

ASCII-only Unicode strings are now accepted by the decoding functions of the
modern interface. For example, ``base64.b64decode('YWJj')`` returns ``b'abc'``.


bz2
---

The :mod:`bz2` module has been rewritten from scratch. In the process, several
new features have been added:

* New :func:`bz2.open` function: open a bzip2-compressed file in binary or
  text mode.

* :class:`bz2.BZ2File` can now read from and write to arbitrary file-like
  objects, by means of its constructor's *fileobj* argument.

  (Contributed by Nadeem Vawda in :issue:`5863`)

* :class:`bz2.BZ2File` and :func:`bz2.decompress` can now decompress
  multi-stream inputs (such as those produced by the :program:`pbzip2` tool).
  :class:`bz2.BZ2File` can now also be used to create this type of file, using
  the ``'a'`` (append) mode.

  (Contributed by Nir Aides in :issue:`1625`)

* :class:`bz2.BZ2File` now implements all of the :class:`io.BufferedIOBase` API,
  except for the :meth:`detach` and :meth:`truncate` methods.


codecs
------

The :mod:`~encodings.mbcs` codec has been rewritten to handle correctly
``replace`` and ``ignore`` error handlers on all Windows versions.  The
:mod:`~encodings.mbcs` codec now supports all error handlers, instead of only
``replace`` to encode and ``ignore`` to decode.

A new Windows-only codec has been added: ``cp65001`` (:issue:`13216`). It is the
Windows code page 65001 (Windows UTF-8, ``CP_UTF8``).  For example, it is used
by ``sys.stdout`` if the console output code page is set to cp65001 (e.g., using
``chcp 65001`` command).

Multibyte CJK decoders now resynchronize faster.  They only ignore the first
byte of an invalid byte sequence. For example, ``b'\xff\n'.decode('gb2312',
'replace')`` now returns a ``\n`` after the replacement character.

(:issue:`12016`)

Incremental CJK codec encoders are no longer reset at each call to their
encode() methods. For example::

    $ ./python -q
    >>> import codecs
    >>> encoder = codecs.getincrementalencoder('hz')('strict')
    >>> b''.join(encoder.encode(x) for x in '\u52ff\u65bd\u65bc\u4eba\u3002 Bye.')
    b'~{NpJ)l6HK!#~} Bye.'

This example gives ``b'~{Np~}~{J)~}~{l6~}~{HK~}~{!#~} Bye.'`` with older Python
versions.

(:issue:`12100`)

The ``unicode_internal`` codec has been deprecated.


collections
-----------

Addition of a new :class:`~collections.ChainMap` class to allow treating a
number of mappings as a single unit.

(Written by Raymond Hettinger for :issue:`11089`, made public in
:issue:`11297`)

The abstract base classes have been moved in a new :mod:`collections.abc`
module, to better differentiate between the abstract and the concrete
collections classes.  Aliases for ABCs are still present in the
:mod:`collections` module to preserve existing imports.

(:issue:`11085`)

.. XXX addition of __slots__ to ABCs not recorded here: internal detail


contextlib
----------

:class:`~contextlib.ExitStack` now provides a solid foundation for
programmatic manipulation of context managers and similar cleanup
functionality. Unlike the previous ``contextlib.nested`` API (which was
deprecated and removed), the new API is designed to work correctly
regardless of whether context managers acquire their resources in
their ``__init__`` method (for example, file objects) or in their
``__enter__`` method (for example, synchronisation objects from the
:mod:`threading` module).

(:issue:`13585`)


crypt
-----

Addition of salt and modular crypt format (hashing method) and the :func:`~crypt.mksalt`
function to the :mod:`crypt` module.

(:issue:`10924`)

curses
------

 * If the :mod:`curses` module is linked to the ncursesw library, use Unicode
   functions when Unicode strings or characters are passed (e.g.
   :c:func:`waddwstr`), and bytes functions otherwise (e.g. :c:func:`waddstr`).
 * Use the locale encoding instead of ``utf-8`` to encode Unicode strings.
 * :class:`curses.window` has a new :attr:`curses.window.encoding` attribute.
 * The :class:`curses.window` class has a new :meth:`~curses.window.get_wch`
   method to get a wide character
 * The :mod:`curses` module has a new :meth:`~curses.unget_wch` function to
   push a wide character so the next :meth:`~curses.window.get_wch` will return
   it

(Contributed by Iñigo Serna in :issue:`6755`)

datetime
--------

 * Equality comparisons between naive and aware :class:`~datetime.datetime`
   instances don't raise :exc:`TypeError`.
 * New :meth:`datetime.datetime.timestamp` method: Return POSIX timestamp
   corresponding to the :class:`~datetime.datetime` instance.
 * The :meth:`datetime.datetime.strftime` method supports formatting years
   older than 1000.
 * XXX The :meth:`datetime.datetime.astimezone` method can now be
   called without arguments to convert datetime instance to the system
   timezone.

decimal
-------

:issue:`7652` - integrate fast native decimal arithmetic.
   C-module and libmpdec written by Stefan Krah.

The new C version of the decimal module integrates the high speed libmpdec
library for arbitrary precision correctly-rounded decimal floating point
arithmetic. libmpdec conforms to IBM's General Decimal Arithmetic Specification.

Performance gains range from 10x for database applications to 100x for
numerically intensive applications. These numbers are expected gains
for standard precisions used in decimal floating point arithmetic. Since
the precision is user configurable, the exact figures may vary. For example,
in integer bignum arithmetic the differences can be significantly higher.

The following table is meant as an illustration. Benchmarks are available
at http://www.bytereef.org/mpdecimal/quickstart.html.

   +---------+-------------+--------------+-------------+
   |         |  decimal.py |   _decimal   |   speedup   |
   +=========+=============+==============+=============+
   |   pi    |    42.02s   |    0.345s    |    120x     |
   +---------+-------------+--------------+-------------+
   | telco   |   172.19s   |    5.68s     |     30x     |
   +---------+-------------+--------------+-------------+
   | psycopg |     3.57s   |    0.29s     |     12x     |
   +---------+-------------+--------------+-------------+

Features
~~~~~~~~

* The :exc:`~decimal.FloatOperation` signal optionally enables stricter
  semantics for mixing floats and Decimals.

* If Python is compiled without threads, the C version automatically
  disables the expensive thread local context machinery. In this case,
  the variable :data:`~decimal.HAVE_THREADS` is set to False.

API changes
~~~~~~~~~~~

* The C module has the following context limits, depending on the machine
  architecture:

   +-------------------+---------------------+------------------------------+
   |                   |       32-bit        |            64-bit            |
   +===================+=====================+==============================+
   | :const:`MAX_PREC` | :const:`425000000`  | :const:`999999999999999999`  |
   +-------------------+---------------------+------------------------------+
   | :const:`MAX_EMAX` | :const:`425000000`  | :const:`999999999999999999`  |
   +-------------------+---------------------+------------------------------+
   | :const:`MIN_EMIN` | :const:`-425000000` | :const:`-999999999999999999` |
   +-------------------+---------------------+------------------------------+

* In the context templates (:class:`~decimal.DefaultContext`,
  :class:`~decimal.BasicContext` and :class:`~decimal.ExtendedContext`)
  the magnitude of :attr:`~decimal.Context.Emax` and
  :attr:`~decimal.Context.Emin` has changed to :const:`999999`.

* The :class:`~decimal.Decimal` constructor in decimal.py does not observe
  the context limits and converts values with arbitrary exponents or precision
  exactly. Since the C version has internal limits, the following scheme is
  used: If possible, values are converted exactly, otherwise
  :exc:`~decimal.InvalidOperation` is raised and the result is NaN. In the
  latter case it is always possible to use :meth:`~decimal.Context.create_decimal`
  in order to obtain a rounded or inexact value.


* The power function in decimal.py is always correctly-rounded. In the
  C version, it is defined in terms of the correctly-rounded
  :meth:`~decimal.Decimal.exp` and :meth:`~decimal.Decimal.ln` functions,
  but the final result is only "almost always correctly rounded".


* In the C version, the context dictionary containing the signals is a
  :class:`~collections.abc.MutableMapping`.  For speed reasons,
  :attr:`~decimal.Context.flags` and :attr:`~decimal.Context.traps` always
  refer to the same :class:`~collections.abc.MutableMapping` that the context
  was initialized with. If a new signal dictionary is assigned,
  :attr:`~decimal.Context.flags` and :attr:`~decimal.Context.traps`
  are updated with the new values, but they do not reference the RHS
  dictionary.


* Pickling a :class:`~decimal.Context` produces a different output in order
  to have a common interchange format for the Python and C versions.


* The order of arguments in the :class:`~decimal.Context` constructor has been
  changed to match the order displayed by :func:`repr`.


* The ``watchexp`` parameter in the :meth:`~decimal.Decimal.quantize` method
  is deprecated.


ftplib
------

* The :class:`~ftplib.FTP_TLS` class now provides a new
  :func:`~ftplib.FTP_TLS.ccc` function to revert control channel back to
  plaintext.  This can be useful to take advantage of firewalls that know how to
  handle NAT with non-secure FTP without opening fixed ports.

  (Contributed by Giampaolo Rodolà in :issue:`12139`)

* Added :meth:`ftplib.FTP.mlsd` method which provides a parsable directory
  listing format and deprecates :meth:`ftplib.FTP.nlst` and
  :meth:`ftplib.FTP.dir`.

  (Contributed by Giampaolo Rodolà in :issue:`11072`)

gc
--

It is now possible to register callbacks invoked by the garbage collector
before and after collection using the new :data:`~gc.callbacks` list.


hmac
----

A new :func:`~hmac.compare_digest` function has been added to prevent
side channel attacks on digests through timing analysis.

(Contributed by Nick Coghlan and Christian Heimes in issue:`15061`)


html.entities
-------------

A new :data:`~html.entities.html5` dictionary that maps HTML5 named character
references to the equivalent Unicode character(s) (e.g. ``html5['gt;'] == '>'``)
has been added to the :mod:`html.entities` module.  The dictionary is now also
used by :class:`~html.parser.HTMLParser`.

(Contributed by Ezio Melotti in :issue:`11113` and :issue:`15156`)


html.parser
-----------

:class:`~html.parser.HTMLParser` is now able to parse broken markup without
raising errors, therefore the *strict* argument of the constructor and the
:exc:`~html.parser.HTMLParseError` exception are now deprecated.
The ability to parse broken markup is the result of a number of bug fixes that
are also available on the latest bug fix releases of Python 2.7/3.2.

(Contributed by Ezio Melotti in :issue:`15114`, and :issue:`14538`,
:issue:`13993`, :issue:`13960`, :issue:`13358`, :issue:`1745761`,
:issue:`755670`, :issue:`13357`, :issue:`12629`, :issue:`1200313`,
:issue:`670664`, :issue:`13273`, :issue:`12888`, :issue:`7311`)

imaplib
-------

The :class:`~imaplib.IMAP4_SSL` constructor now accepts an SSLContext
parameter to control parameters of the secure channel.

(Contributed by Sijin Joseph in :issue:`8808`)


inspect
-------

A new :func:`~inspect.getclosurevars` function has been added. This function
reports the current binding of all names referenced from the function body and
where those names were resolved, making it easier to verify correct internal
state when testing code that relies on stateful closures.

(Contributed by Meador Inge and Nick Coghlan in :issue:`13062`)

A new :func:`~inspect.getgeneratorlocals` function has been added. This
function reports the current binding of local variables in the generator's
stack frame, making it easier to verify correct internal state when testing
generators.

(Contributed by Meador Inge in :issue:`15153`)

io
--

The :func:`~io.open` function has a new ``'x'`` mode that can be used to
exclusively create a new file, and raise a :exc:`FileExistsError` if the file
already exists. It is based on the C11 'x' mode to fopen().

(Contributed by David Townshend in :issue:`12760`)

The constructor of the :class:`~io.TextIOWrapper` class has a new
*write_through* optional argument. If *write_through* is ``True``, calls to
:meth:`~io.TextIOWrapper.write` are guaranteed not to be buffered: any data
written on the :class:`~io.TextIOWrapper` object is immediately handled to its
underlying binary buffer.


math
----

The :mod:`math` module has a new function:

  * :func:`~math.log2`: return the base-2 logarithm of *x*
    (Written by Mark Dickinson in :issue:`11888`).


multiprocessing
---------------

The new :func:`multiprocessing.connection.wait` function allows to poll
multiple objects (such as connections, sockets and pipes) with a timeout.
(Contributed by Richard Oudkerk in :issue:`12328`.)

:class:`multiprocessing.Connection` objects can now be transferred over
multiprocessing connections.
(Contributed by Richard Oudkerk in :issue:`4892`.)


nntplib
-------

The :class:`nntplib.NNTP` class now supports the context manager protocol to
unconditionally consume :exc:`socket.error` exceptions and to close the NNTP
connection when done::

  >>> from nntplib import NNTP
  >>> with NNTP('news.gmane.org') as n:
  ...     n.group('gmane.comp.python.committers')
  ...
  ('211 1755 1 1755 gmane.comp.python.committers', 1755, 1, 1755, 'gmane.comp.python.committers')
  >>>

(Contributed by Giampaolo Rodolà in :issue:`9795`)


os
--

* The :mod:`os` module has a new :func:`~os.pipe2` function that makes it
  possible to create a pipe with :data:`~os.O_CLOEXEC` or
  :data:`~os.O_NONBLOCK` flags set atomically. This is especially useful to
  avoid race conditions in multi-threaded programs.

* The :mod:`os` module has a new :func:`~os.sendfile` function which provides
  an efficent "zero-copy" way for copying data from one file (or socket)
  descriptor to another. The phrase "zero-copy" refers to the fact that all of
  the copying of data between the two descriptors is done entirely by the
  kernel, with no copying of data into userspace buffers. :func:`~os.sendfile`
  can be used to efficiently copy data from a file on disk to a network socket,
  e.g. for downloading a file.

  (Patch submitted by Ross Lagerwall and Giampaolo Rodolà in :issue:`10882`.)

* To avoid race conditions like symlink attacks and issues with temporary
  files and directories, it is more reliable (and also faster) to manipulate
  file descriptors instead of file names. Python 3.3 enhances existing functions
  and introduces new functions to work on file descriptors (:issue:`4761`,
  :issue:`10755` and :issue:`14626`).

  - The :mod:`os` module has a new :func:`~os.fwalk` function similar to
    :func:`~os.walk` except that it also yields file descriptors referring to the
    directories visited. This is especially useful to avoid symlink races.

  - The following functions get new optional *dir_fd* (:ref:`paths relative to
    directory descriptors <dir_fd>`) and/or *follow_symlinks* (:ref:`not
    following symlinks <follow_symlinks>`):
    :func:`~os.access`, :func:`~os.chflags`, :func:`~os.chmod`, :func:`~os.chown`,
    :func:`~os.link`, :func:`~os.lstat`, :func:`~os.mkdir`, :func:`~os.mkfifo`,
    :func:`~os.mknod`, :func:`~os.open`, :func:`~os.readlink`, :func:`~os.remove`,
    :func:`~os.rename`, :func:`~os.replace`, :func:`~os.rmdir`, :func:`~os.stat`,
    :func:`~os.symlink`, :func:`~os.unlink`, :func:`~os.utime`.

  - The following functions now support a file descriptor for their path argument:
    :func:`~os.chdir`, :func:`~os.chmod`, :func:`~os.chown`,
    :func:`~os.execve`, :func:`~os.listdir`, :func:`~os.pathconf`, :func:`~os.path.exists`,
    :func:`~os.stat`, :func:`~os.statvfs`, :func:`~os.utime`.

* The :mod:`os` module has two new functions: :func:`~os.getpriority` and
  :func:`~os.setpriority`. They can be used to get or set process
  niceness/priority in a fashion similar to :func:`os.nice` but extended to all
  processes instead of just the current one.

  (Patch submitted by Giampaolo Rodolà in :issue:`10784`.)

* The new :func:`os.replace` function allows cross-platform renaming of a
  file with overwriting the destination.  With :func:`os.rename`, an existing
  destination file is overwritten under POSIX, but raises an error under
  Windows.
  (Contributed by Antoine Pitrou in :issue:`8828`.)

* The stat family of functions (:func:`~os.stat`, :func:`~os.fstat`,
  and :func:`~os.lstat`) now support reading a file's timestamps
  with nanosecond precision.  Symmetrically, :func:`~os.utime`
  can now write file timestamps with nanosecond precision.  (Contributed by
  Larry Hastings in :issue:`14127`.)

* The new :func:`os.get_terminal_size` function queries the size of the
  terminal attached to a file descriptor. See also
  :func:`shutil.get_terminal_size`.
  (Contributed by Zbigniew Jędrzejewski-Szmek in :issue:`13609`.)

.. XXX sort out this mess after beta1

* New functions to support Linux extended attributes (:issue:`12720`):
  :func:`~os.getxattr`, :func:`~os.listxattr`, :func:`~os.removexattr`,
  :func:`~os.setxattr`.

* New interface to the scheduler. These functions
  control how a process is allocated CPU time by the operating system. New
  functions:
  :func:`~os.sched_get_priority_max`, :func:`~os.sched_get_priority_min`,
  :func:`~os.sched_getaffinity`, :func:`~os.sched_getparam`,
  :func:`~os.sched_getscheduler`, :func:`~os.sched_rr_get_interval`,
  :func:`~os.sched_setaffinity`, :func:`~os.sched_setparam`,
  :func:`~os.sched_setscheduler`, :func:`~os.sched_yield`,

* New functions to control the file system:

  * :func:`~os.posix_fadvise`: Announces an intention to access data in a
    specific pattern thus allowing the kernel to make optimizations.
  * :func:`~os.posix_fallocate`: Ensures that enough disk space is allocated
    for a file.
  * :func:`~os.sync`: Force write of everything to disk.

* Add some extra posix functions to the os module:

  * :func:`~os.lockf`: Apply, test or remove a POSIX lock on an open file descriptor.
  * :func:`~os.pread`: Read from a file descriptor at an offset, the file
    offset remains unchanged.
  * :func:`~os.pwrite`: Write to a file descriptor from an offset, leaving
    the file offset unchanged.
  * :func:`~os.readv`: Read from a file descriptor into a number of writable buffers.
  * :func:`~os.truncate`: Truncate the file corresponding to *path*, so that
    it is at most *length* bytes in size.
  * :func:`~os.waitid`: Wait for the completion of one or more child processes.
  * :func:`~os.writev`: Write the contents of *buffers* to a file descriptor,
    where *buffers* is an arbitrary sequence of buffers.
  * :func:`~os.getgrouplist` (:issue:`9344`): Return list of group ids that
    specified user belongs to.

* :func:`~os.times` and :func:`~os.uname`: Return type changed from a tuple to
  a tuple-like object with named attributes.


pdb
---

* Tab-completion is now available not only for command names, but also their
  arguments.  For example, for the ``break`` command, function and file names
  are completed.  (Contributed by Georg Brandl in :issue:`14210`)


pickle
------

:class:`pickle.Pickler` objects now have an optional
:attr:`~pickle.Pickler.dispatch_table` attribute allowing to set per-pickler
reduction functions.
(Contributed by Richard Oudkerk in :issue:`14166`.)


pydoc
-----

The Tk GUI and the :func:`~pydoc.serve` function have been removed from the
:mod:`pydoc` module: ``pydoc -g`` and :func:`~pydoc.serve` have been deprecated
in Python 3.2.


re
--

:class:`str` regular expressions now support ``\u`` and ``\U`` escapes.

(Contributed by Serhiy Storchaka in :issue:`3665`.)


sched
-----

* :meth:`~sched.scheduler.run` now accepts a *blocking* parameter which when
  set to False makes the method execute the scheduled events due to expire
  soonest (if any) and then return immediately.
  This is useful in case you want to use the :class:`~sched.scheduler` in
  non-blocking applications.  (Contributed by Giampaolo Rodolà in :issue:`13449`)

* :class:`~sched.scheduler` class can now be safely used in multi-threaded
  environments.  (Contributed by Josiah Carlson and Giampaolo Rodolà in
  :issue:`8684`)

* *timefunc* and *delayfunct* parameters of :class:`~sched.scheduler` class
  constructor are now optional and defaults to :func:`time.time` and
  :func:`time.sleep` respectively.  (Contributed by Chris Clark in
  :issue:`13245`)

* :meth:`~sched.scheduler.enter` and :meth:`~sched.scheduler.enterabs`
  *argument* parameter is now optional.  (Contributed by Chris Clark in
  :issue:`13245`)

* :meth:`~sched.scheduler.enter` and :meth:`~sched.scheduler.enterabs`
  now accept a *kwargs* parameter.  (Contributed by Chris Clark in
  :issue:`13245`)


shutil
------

* The :mod:`shutil` module has these new fuctions:

  * :func:`~shutil.disk_usage`: provides total, used and free disk space
    statistics. (Contributed by Giampaolo Rodolà in :issue:`12442`)
  * :func:`~shutil.chown`: allows one to change user and/or group of the given
    path also specifying the user/group names and not only their numeric
    ids. (Contributed by Sandro Tosi in :issue:`12191`)

* :func:`~shutil.copy2` and :func:`~shutil.copystat` now preserve file
  timestamps with nanosecond precision on platforms that support it.
  They also preserve file "extended attributes" on Linux.  (Contributed
  by Larry Hastings in :issue:`14127` and  :issue:`15238`.)

* The new :func:`shutil.get_terminal_size` function returns the size of the
  terminal window the interpreter is attached to.
  (Contributed by Zbigniew Jędrzejewski-Szmek in :issue:`13609`.)

* Several functions now take an optional ``symlinks`` argument: when that
  parameter is true, symlinks aren't dereferenced and the operation instead
  acts on the symlink itself (or creates one, if relevant).
  (Contributed by Hynek Schlawack in :issue:`12715`.)

* :func:`~shutil.rmtree` is now resistant to symlink attacks on platforms
  which support the new ``dir_fd`` parameter in :func:`os.open` and
  :func:`os.unlink`. (Contributed by Martin von Löwis and Hynek Schlawack
  in :issue:`4489`.)



signal
------

* The :mod:`signal` module has new functions:

  * :func:`~signal.pthread_sigmask`: fetch and/or change the signal mask of the
    calling thread (Contributed by Jean-Paul Calderone in :issue:`8407`) ;
  * :func:`~signal.pthread_kill`: send a signal to a thread ;
  * :func:`~signal.sigpending`: examine pending functions ;
  * :func:`~signal.sigwait`: wait a signal.
  * :func:`~signal.sigwaitinfo`: wait for a signal, returning detailed
    information about it.
  * :func:`~signal.sigtimedwait`: like :func:`~signal.sigwaitinfo` but with a
    timeout.

* The signal handler writes the signal number as a single byte instead of
  a nul byte into the wakeup file descriptor. So it is possible to wait more
  than one signal and know which signals were raised.

* :func:`signal.signal` and :func:`signal.siginterrupt` raise an OSError,
  instead of a RuntimeError: OSError has an errno attribute.

smtplib
-------

The :class:`~smtplib.SMTP_SSL` constructor and the :meth:`~smtplib.SMTP.starttls`
method now accept an SSLContext parameter to control parameters of the secure
channel.

(Contributed by Kasun Herath in :issue:`8809`)


socket
------

* The :class:`~socket.socket` class now exposes additional methods to process
  ancillary data when supported by the underlying platform:

  * :func:`~socket.socket.sendmsg`
  * :func:`~socket.socket.recvmsg`
  * :func:`~socket.socket.recvmsg_into`

  (Contributed by David Watson in :issue:`6560`, based on an earlier patch by
  Heiko Wundram)

* The :class:`~socket.socket` class now supports the PF_CAN protocol family
  (http://en.wikipedia.org/wiki/Socketcan), on Linux
  (http://lwn.net/Articles/253425).

  (Contributed by Matthias Fuchs, updated by Tiago Gonçalves in :issue:`10141`)

* The :class:`~socket.socket` class now supports the PF_RDS protocol family
  (http://en.wikipedia.org/wiki/Reliable_Datagram_Sockets and
  http://oss.oracle.com/projects/rds/).


ssl
---

* The :mod:`ssl` module has two new random generation functions:

  * :func:`~ssl.RAND_bytes`: generate cryptographically strong
    pseudo-random bytes.
  * :func:`~ssl.RAND_pseudo_bytes`: generate pseudo-random bytes.

  (Contributed by Victor Stinner in :issue:`12049`)

* The :mod:`ssl` module now exposes a finer-grained exception hierarchy
  in order to make it easier to inspect the various kinds of errors.

  (Contributed by Antoine Pitrou in :issue:`11183`)

* :meth:`~ssl.SSLContext.load_cert_chain` now accepts a *password* argument
  to be used if the private key is encrypted.

  (Contributed by Adam Simpkins in :issue:`12803`)

* Diffie-Hellman key exchange, both regular and Elliptic Curve-based, is
  now supported through the :meth:`~ssl.SSLContext.load_dh_params` and
  :meth:`~ssl.SSLContext.set_ecdh_curve` methods.

  (Contributed by Antoine Pitrou in :issue:`13626` and :issue:`13627`)

* SSL sockets have a new :meth:`~ssl.SSLSocket.get_channel_binding` method
  allowing the implementation of certain authentication mechanisms such as
  SCRAM-SHA-1-PLUS.

  (Contributed by Jacek Konieczny in :issue:`12551`)

* You can query the SSL compression algorithm used by an SSL socket, thanks
  to its new :meth:`~ssl.SSLSocket.compression` method.

  (Contributed by Antoine Pitrou in :issue:`13634`)

* Support has been added for the Next Procotol Negotiation extension using
  the :meth:`ssl.SSLContext.set_npn_protocols` method.

  (Contributed by Colin Marc in :issue:`14204`)

* SSL errors can now be introspected more easily thanks to
  :attr:`~ssl.SSLError.library` and :attr:`~ssl.SSLError.reason` attributes.

  (Contributed by Antoine Pitrou in :issue:`14837`)

stat
----

- The undocumented tarfile.filemode function has been moved to
  :func:`stat.filemode`. It can be used to convert a file's mode to a string of
  the form '-rwxrwxrwx'.

  (Contributed by Giampaolo Rodolà in :issue:`14807`)

sys
---

* The :mod:`sys` module has a new :data:`~sys.thread_info` :term:`struct
  sequence` holding informations about the thread implementation.

  (:issue:`11223`)

textwrap
--------

* The :mod:`textwrap` module has a new :func:`~textwrap.indent` that makes
  it straightforward to add a common prefix to selected lines in a block
  of text.

  (:issue:`13857`)

time
----

The :pep:`418` added new functions to the :mod:`time` module:

* :func:`~time.get_clock_info`: Get information on a clock.
* :func:`~time.monotonic`: Monotonic clock (cannot go backward), not affected
  by system clock updates.
* :func:`~time.perf_counter`: Performance counter with the highest available
  resolution to measure a short duration.
* :func:`~time.process_time`: Sum of the system and user CPU time of the
  current process.

Other new functions:

* :func:`~time.clock_getres`, :func:`~time.clock_gettime` and
  :func:`~time.clock_settime` functions with ``CLOCK_xxx`` constants.
  (Contributed by Victor Stinner in :issue:`10278`)


types
-----

Add a new :class:`types.MappingProxyType` class: Read-only proxy of a mapping.
(:issue:`14386`)


The new functions `types.new_class` and `types.prepare_class` provide support
for PEP 3115 compliant dynamic type creation. (:issue:`14588`)


unittest
--------

:meth:`.assertRaises`, :meth:`.assertRaisesRegex`, :meth:`.assertWarns`, and
:meth:`.assertWarnsRegex` now accept a keyword argument *msg* when used as
context managers.

(Contributed by Ezio Melotti and Winston Ewert in :issue:`10775`)


urllib
------

The :class:`~urllib.request.Request` class, now accepts a *method* argument
used by :meth:`~urllib.request.Request.get_method` to determine what HTTP method
should be used.  For example, this will send a ``'HEAD'`` request::

   >>> urlopen(Request('http://www.python.org', method='HEAD'))

(:issue:`1673007`)


webbrowser
----------

The :mod:`webbrowser` module supports more browsers: Google Chrome (named
:program:`chrome`, :program:`chromium`, :program:`chrome-browser` or
:program:`chromium-browser` depending on the version and operating system) as
well as the the generic launchers :program:`xdg-open` from the FreeDesktop.org
project and :program:`gvfs-open` which is the default URI handler for GNOME 3.

(:issue:`13620` and :issue:`14493`)


xml.etree.ElementTree
---------------------

The :mod:`xml.etree.ElementTree` module now imports its C accelerator by
default; there is no longer a need to explicitly import
:mod:`xml.etree.cElementTree` (this module stays for backwards compatibility,
but is now deprecated).  In addition,  the ``iter`` family of methods of
:class:`~xml.etree.ElementTree.Element` has been optimized (rewritten in C).
The module's documentation has also been greatly improved with added examples
and a more detailed reference.


Optimizations
=============

Major performance enhancements have been added:

* Thanks to :pep:`393`, some operations on Unicode strings have been optimized:

  * the memory footprint is divided by 2 to 4 depending on the text
  * encode an ASCII string to UTF-8 doesn't need to encode characters anymore,
    the UTF-8 representation is shared with the ASCII representation
  * the UTF-8 encoder has been optimized
  * repeating a single ASCII letter and getting a substring of a ASCII strings
    is 4 times faster

* UTF-8 is now 2x to 4x faster.  UTF-16 encoding is now up to 10x faster.

  (contributed by Serhiy Storchaka, :issue:`14624`, :issue:`14738` and
  :issue:`15026`.)


Build and C API Changes
=======================

Changes to Python's build process and to the C API include:

* New :pep:`3118` related function:

  * :c:func:`PyMemoryView_FromMemory`

* :pep:`393` added new Unicode types, macros and functions:

  * High-level API:

    * :c:func:`PyUnicode_CopyCharacters`
    * :c:func:`PyUnicode_FindChar`
    * :c:func:`PyUnicode_GetLength`, :c:macro:`PyUnicode_GET_LENGTH`
    * :c:func:`PyUnicode_New`
    * :c:func:`PyUnicode_Substring`
    * :c:func:`PyUnicode_ReadChar`, :c:func:`PyUnicode_WriteChar`

  * Low-level API:

    * :c:type:`Py_UCS1`, :c:type:`Py_UCS2`, :c:type:`Py_UCS4` types
    * :c:type:`PyASCIIObject` and :c:type:`PyCompactUnicodeObject` structures
    * :c:macro:`PyUnicode_READY`
    * :c:func:`PyUnicode_FromKindAndData`
    * :c:func:`PyUnicode_AsUCS4`, :c:func:`PyUnicode_AsUCS4Copy`
    * :c:macro:`PyUnicode_DATA`, :c:macro:`PyUnicode_1BYTE_DATA`,
      :c:macro:`PyUnicode_2BYTE_DATA`, :c:macro:`PyUnicode_4BYTE_DATA`
    * :c:macro:`PyUnicode_KIND` with :c:type:`PyUnicode_Kind` enum:
      :c:data:`PyUnicode_WCHAR_KIND`, :c:data:`PyUnicode_1BYTE_KIND`,
      :c:data:`PyUnicode_2BYTE_KIND`, :c:data:`PyUnicode_4BYTE_KIND`
    * :c:macro:`PyUnicode_READ`, :c:macro:`PyUnicode_READ_CHAR`, :c:macro:`PyUnicode_WRITE`
    * :c:macro:`PyUnicode_MAX_CHAR_VALUE`



Deprecated
==========

Unsupported Operating Systems
-----------------------------

OS/2 and VMS are no longer supported due to the lack of a maintainer.

Windows 2000 and Windows platforms which set ``COMSPEC`` to ``command.com``
are no longer supported due to maintenance burden.


Deprecated Python modules, functions and methods
------------------------------------------------

* The ``unicode_internal`` codec has been deprecated because of the
  :pep:`393`, use UTF-8, UTF-16 (``utf-16-le`` or ``utf-16-be``), or UTF-32
  (``utf-32-le`` or ``utf-32-be``)
* :meth:`ftplib.FTP.nlst` and :meth:`ftplib.FTP.dir`: use
  :meth:`ftplib.FTP.mlsd`
* :func:`platform.popen`: use the :mod:`subprocess` module. Check especially
  the :ref:`subprocess-replacements` section.
* :issue:`13374`: The Windows bytes API has been deprecated in the :mod:`os`
  module. Use Unicode filenames, instead of bytes filenames, to not depend on
  the ANSI code page anymore and to support any filename.
* :issue:`13988`: The :mod:`xml.etree.cElementTree` module is deprecated.  The
  accelerator is used automatically whenever available.
* The behaviour of :func:`time.clock` depends on the platform: use the new
  :func:`time.perf_counter` or :func:`time.process_time` function instead,
  depending on your requirements, to have a well defined behaviour.
* The :func:`os.stat_float_times` function is deprecated.
* :mod:`abc` module:

  * :class:`abc.abstractproperty` has been deprecated, use :class:`property`
    with :func:`abc.abstractmethod` instead.
  * :class:`abc.abstractclassmethod` has been deprecated, use
    :class:`classmethod` with :func:`abc.abstractmethod` instead.
  * :class:`abc.abstractstaticmethod` has been deprecated, use
    :class:`staticmethod` with :func:`abc.abstractmethod` instead.

* :mod:`importlib` package:

  * :meth:`importlib.abc.SourceLoader.path_mtime` is now deprecated in favour of
    :meth:`importlib.abc.SourceLoader.path_stats` as bytecode files now store
    both the modification time and size of the source file the bytecode file was
    compiled from.





Deprecated functions and types of the C API
-------------------------------------------

The :c:type:`Py_UNICODE` has been deprecated by :pep:`393` and will be
removed in Python 4. All functions using this type are deprecated:

Unicode functions and methods using :c:type:`Py_UNICODE` and
:c:type:`Py_UNICODE*` types:

 * :c:macro:`PyUnicode_FromUnicode`: use :c:func:`PyUnicode_FromWideChar` or
   :c:func:`PyUnicode_FromKindAndData`
 * :c:macro:`PyUnicode_AS_UNICODE`, :c:func:`PyUnicode_AsUnicode`,
   :c:func:`PyUnicode_AsUnicodeAndSize`: use :c:func:`PyUnicode_AsWideCharString`
 * :c:macro:`PyUnicode_AS_DATA`: use :c:macro:`PyUnicode_DATA` with
   :c:macro:`PyUnicode_READ` and :c:macro:`PyUnicode_WRITE`
 * :c:macro:`PyUnicode_GET_SIZE`, :c:func:`PyUnicode_GetSize`: use
   :c:macro:`PyUnicode_GET_LENGTH` or :c:func:`PyUnicode_GetLength`
 * :c:macro:`PyUnicode_GET_DATA_SIZE`: use
   ``PyUnicode_GET_LENGTH(str) * PyUnicode_KIND(str)`` (only work on ready
   strings)
 * :c:func:`PyUnicode_AsUnicodeCopy`: use :c:func:`PyUnicode_AsUCS4Copy` or
   :c:func:`PyUnicode_AsWideCharString`
 * :c:func:`PyUnicode_GetMax`


Functions and macros manipulating Py_UNICODE* strings:

 * :c:macro:`Py_UNICODE_strlen`: use :c:func:`PyUnicode_GetLength` or
   :c:macro:`PyUnicode_GET_LENGTH`
 * :c:macro:`Py_UNICODE_strcat`: use :c:func:`PyUnicode_CopyCharacters` or
   :c:func:`PyUnicode_FromFormat`
 * :c:macro:`Py_UNICODE_strcpy`, :c:macro:`Py_UNICODE_strncpy`,
   :c:macro:`Py_UNICODE_COPY`: use :c:func:`PyUnicode_CopyCharacters` or
   :c:func:`PyUnicode_Substring`
 * :c:macro:`Py_UNICODE_strcmp`: use :c:func:`PyUnicode_Compare`
 * :c:macro:`Py_UNICODE_strncmp`: use :c:func:`PyUnicode_Tailmatch`
 * :c:macro:`Py_UNICODE_strchr`, :c:macro:`Py_UNICODE_strrchr`: use
   :c:func:`PyUnicode_FindChar`
 * :c:macro:`Py_UNICODE_FILL`: use :c:func:`PyUnicode_Fill`
 * :c:macro:`Py_UNICODE_MATCH`

Encoders:

 * :c:func:`PyUnicode_Encode`: use :c:func:`PyUnicode_AsEncodedObject`
 * :c:func:`PyUnicode_EncodeUTF7`
 * :c:func:`PyUnicode_EncodeUTF8`: use :c:func:`PyUnicode_AsUTF8` or
   :c:func:`PyUnicode_AsUTF8String`
 * :c:func:`PyUnicode_EncodeUTF32`
 * :c:func:`PyUnicode_EncodeUTF16`
 * :c:func:`PyUnicode_EncodeUnicodeEscape:` use
   :c:func:`PyUnicode_AsUnicodeEscapeString`
 * :c:func:`PyUnicode_EncodeRawUnicodeEscape:` use
   :c:func:`PyUnicode_AsRawUnicodeEscapeString`
 * :c:func:`PyUnicode_EncodeLatin1`: use :c:func:`PyUnicode_AsLatin1String`
 * :c:func:`PyUnicode_EncodeASCII`: use :c:func:`PyUnicode_AsASCIIString`
 * :c:func:`PyUnicode_EncodeCharmap`
 * :c:func:`PyUnicode_TranslateCharmap`
 * :c:func:`PyUnicode_EncodeMBCS`: use :c:func:`PyUnicode_AsMBCSString` or
   :c:func:`PyUnicode_EncodeCodePage` (with ``CP_ACP`` code_page)
 * :c:func:`PyUnicode_EncodeDecimal`,
   :c:func:`PyUnicode_TransformDecimalToASCII`


Deprecated features
-------------------

The :mod:`array` module's ``'u'`` format code is now deprecated and will be
removed in Python 4 together with the rest of the (:c:type:`Py_UNICODE`) API.


Porting to Python 3.3
=====================

This section lists previously described changes and other bugfixes
that may require changes to your code.

.. _portingpythoncode:

Porting Python code
-------------------

* Hash randomization is enabled by default. Set the :envvar:`PYTHONHASHSEED`
  environment variable to ``0`` to disable hash randomization. See also the
  :meth:`object.__hash__` method.

* :issue:`12326`: On Linux, sys.platform doesn't contain the major version
  anymore. It is now always 'linux', instead of 'linux2' or 'linux3' depending
  on the Linux version used to build Python. Replace sys.platform == 'linux2'
  with sys.platform.startswith('linux'), or directly sys.platform == 'linux' if
  you don't need to support older Python versions.

* :issue:`13847`, :issue:`14180`: :mod:`time` and :mod:`datetime`:
  :exc:`OverflowError` is now raised instead of :exc:`ValueError` if a
  timestamp is out of range. :exc:`OSError` is now raised if C functions
  :c:func:`gmtime` or :c:func:`localtime` failed.

* The default finders used by import now utilize a cache of what is contained
  within a specific directory. If you create a Python source file or sourceless
  bytecode file, make sure to call :func:`importlib.invalidate_caches` to clear
  out the cache for the finders to notice the new file.

* :exc:`ImportError` now uses the full name of the module that was attemped to
  be imported. Doctests that check ImportErrors' message will need to be
  updated to use the full name of the module instead of just the tail of the
  name.

* The *index* argument to :func:`__import__` now defaults to 0 instead of -1
  and no longer support negative values. It was an oversight when :pep:`328` was
  implemented that the default value remained -1. If you need to continue to
  perform a relative import followed by an absolute import, then perform the
  relative import using an index of 1, followed by another import using an
  index of 0. It is preferred, though, that you use
  :func:`importlib.import_module` rather than call :func:`__import__` directly.

* :func:`__import__` no longer allows one to use an index value other than 0
  for top-level modules. E.g. ``__import__('sys', level=1)`` is now an error.

* Because :attr:`sys.meta_path` and :attr:`sys.path_hooks` now have finders on
  them by default, you will most likely want to use :meth:`list.insert` instead
  of :meth:`list.append` to add to those lists.

* Because ``None`` is now inserted into :attr:`sys.path_importer_cache`, if you
  are clearing out entries in the dictionary of paths that do not have a
  finder, you will need to remove keys paired with values of ``None`` **and**
  :class:`imp.NullImporter` to be backwards-compatible. This will lead to extra
  overhead on older versions of Python that re-insert ``None`` into
  :attr:`sys.path_importer_cache` where it repesents the use of implicit
  finders, but semantically it should not change anything.

* :class:`importlib.abc.Finder` no longer specifies a `find_module()` abstract
  method that must be implemented. If you were relying on subclasses to
  implement that method, make sure to check for the method's existence first.
  You will probably want to check for `find_loader()` first, though, in the
  case of working with :term:`path entry finders <path entry finder>`.

* :mod:`pkgutil` has been converted to use :mod:`importlib` internally. This
  eliminates many edge cases where the old behaviour of the PEP 302 import
  emulation failed to match the behaviour of the real import system. The
  import emulation itself is still present, but is now deprecated. The
  :func:`pkgutil.iter_importers` and :func:`pkgutil.walk_packages` functions
  special case the standard import hooks so they are still supported even
  though they do not provide the non-standard ``iter_modules()`` method.


Porting C code
--------------

* In the course of changes to the buffer API the undocumented
  :c:member:`~Py_buffer.smalltable` member of the
  :c:type:`Py_buffer` structure has been removed and the
  layout of the :c:type:`PyMemoryViewObject` has changed.

  All extensions relying on the relevant parts in ``memoryobject.h``
  or ``object.h`` must be rebuilt.

* Due to :ref:`PEP 393 <pep-393>`, the :c:type:`Py_UNICODE` type and all
  functions using this type are deprecated (but will stay available for
  at least five years).  If you were using low-level Unicode APIs to
  construct and access unicode objects and you want to benefit of the
  memory footprint reduction provided by PEP 393, you have to convert
  your code to the new :doc:`Unicode API <../c-api/unicode>`.

  However, if you only have been using high-level functions such as
  :c:func:`PyUnicode_Concat()`, :c:func:`PyUnicode_Join` or
  :c:func:`PyUnicode_FromFormat()`, your code will automatically take
  advantage of the new unicode representations.

* :c:func:`PyImport_GetMagicNumber` now returns -1 upon failure.

* As a negative value for the *level* argument to :func:`__import__` is no
  longer valid, the same now holds for :c:func:`PyImport_ImportModuleLevel`.
  This also means that the value of *level* used by
  :c:func:`PyImport_ImportModuleEx` is now 0 instead of -1.


Building C extensions
---------------------

* The range of possible file names for C extensions has been narrowed.
  Very rarely used spellings have been suppressed: under POSIX, files
  named ``xxxmodule.so``, ``xxxmodule.abi3.so`` and
  ``xxxmodule.cpython-*.so`` are no longer recognized as implementing
  the ``xxx`` module.  If you had been generating such files, you have
  to switch to the other spellings (i.e., remove the ``module`` string
  from the file names).

  (implemented in :issue:`14040`.)


Other issues
------------

.. Issue #11591: When :program:`python` was started with :option:`-S`,
   ``import site`` will not add site-specific paths to the module search
   paths.  In previous versions, it did.  See changeset for doc changes in
   various files.  Contributed by Carl Meyer with editions by Éric Araujo.

.. Issue #10998: the -Q command-line flag and related artifacts have been
   removed.  Code checking sys.flags.division_warning will need updating.
   Contributed by Éric Araujo.